There is a well-known tradeoff between high speed and sensitivity in a photodetector. High bandwidth signal detection requires a short transit time of the carriers and thus a thin absorption layer. However, the geometrical constraints on the absorption layer thickness results in a reduced absorption and lower responsivity.
One type of semiconductor-based photodetector is termed a p-i-n junction diode, or a PIN diode. This type of structure is generally composed of a number of solid semiconductive sandwiched together in an epitaxial structure. In particular, a p-type semiconductor material and an n-type semiconductor region are separated by an intrinsic semiconductor.
In a PIN diode, the depletion layer extends into each side of junction by a distance that is inversely proportional to the doping concentration. Thus, the p-i depletion layer extends well into the intrinsic material, as does the depletion layer of the i-n junction. Accordingly, a PIN diode functions like a p-n junction with a depletion layer that encompasses the entirety of the intrinsic material. The primary advantages inherent to this structure are twofold. First, the addition of the intrinsic layer permits a fractional increase in the amount of light to be captured by the diode. Secondly, due to the extended depletion layer, the PIN diode has a very small junction capacitance and corresponding fast response.
Most attempts at increasing the speed of PIN diodes have focused on reducing the capacitance at the junction. At least one proposed design has included an undoped drift region for this purpose, effectively increasing the size of the intrinsic portion of the diode. Although this solution is suitable for decreasing the junction capacitance, it unfortunately increases the transit time for the carriers and thus reduces the response time of the photodetector. As such, there is a need in the art for an improved photodetector that strikes the proper balance between capacitance and response time, while increasing the responsivity of the device.
Accordingly, the present invention includes a photodiode having a first p-type semiconductor layer and an n-type semiconductor layer coupled by a second p-type semiconductor layer. The second p-type semiconductor layer has graded doping along the path of the carriers. In particular, the doping is concentration graded from a high value near the anode to a lower p concentration towards the cathode. By grading the doping in this way, an increase in absorption is achieved, improving the responsivity of the device. Although this doping increases the capacitance relative to an intrinsic semiconductor of the same thickness, the pseudo electric field that is created by the graded doping gives the electrons a very high velocity which more than compensates for this increased capacitance. Further embodiments and advantages of the present invention are discussed below with reference to the figures.